Thirty years ago, in The New York Review of Books, I reviewed a pioneering work of what was to become the new animal rights movement. The book was a collection of essays called Animals, Men and Morals. I headed my review "Animal Liberation", a title that invited - and received - ridicule. But I used it deliberately, to say that just as we needed to overcome prejudices against black people, women and gays, so too we should strive to overcome our prejudices against non-human animals and start taking their interests seriously.

I did not deny or minimise the evident differences between humans and animals, but I argued that these differences do not justify the way we think of, or treat, animals. Being able to reason better than another being doesn't mean that our pains and pleasures count more than those of others - whether those "others" are human or non-human. After all, some humans - infants and those with severe intellectual disabilities - don't reason as well as some non-human animals, but we would, rightly, be shocked by anyone who proposed that we inflict slow, painful deaths on these intellectually inferior humans to test the safety of household products. Nor, of course, would we tolerate confining them in small cages and then slaughtering them in order to eat them. The fact that we are prepared to do these things to non-human animals is therefore a sign of "speciesism", a prejudice that survives because it is convenient for the dominant group - in this case, not whites or males, but all humans.

A lot has changed since the appearance of that review and of the book, also called Animal Liberation, that grew out of it. We have seen the development of an entirely new movement that has had a significant impact on the way many people think about animals. A voluminous literature on animals and ethics has sprung up, and vigorous philosophical debate continues. One of the most significant developments is how science has come to the aid of the animal movement. This may seem odd, since animal advocates who criticise the use of animals in research are likely to be painted as "anti-science". But the animal movement must take its stand firmly on the side of science - a science bound by ethical constraints on how it treats animals, just as it is bound by ethical constraints on the way it treats human subjects of research.

Science assists the animal movement in many ways. Evolutionary theory effectively debunks the idea that God gave humans dominion over the other animals - used for millennia as an excuse for doing as we please with them. Now the mapping and sequencing of the human genome and of the genome of our close relative the chimpanzee, is showing us just how closely related we are. Next week, the prestigious American journal Proceedings of the National Academy of Sciences will publish an article arguing, on strictly scientific grounds, that chimpanzees should be included in the genus "homo", hitherto reserved for humans. To people used to dividing the world up into "humans" and "animals" it comes as a shock to realise that the genetic differences between humans and chimpanzees are smaller than those between chimpanzees and gorillas. We have also learned, thanks to the work of Jane Goodall and many other scientists, how rich and complex the social and emotional lives of chimpanzees and other great apes are. Why, then, do we continue to insist that even the most basic rights, like those to life, liberty and protection from torture, are for humans only?

But it is not only with the species closest to humans that scientific research is providing insights into lives. Lynne Sneddon and other scientists at the Roslin Institute have given new life to the case against fishing, providing strong evidence that fish feel pain and that being hooked in the lips is painful to them. Though the research caused distress to fish, if it helps to turn the tide of public opinion against the "sport" of angling, it will have reduced the total amount of pain fish experience by millions of times that which the researchers caused.

A similar calculation has already justified scientific research using farm animals to ascertain their attitude to different forms of housing on intensive farms, like battery cages for hens and individual stalls for breeding sows. The hens, for example, were made familiar with cages and then with an outside run. Not surprisingly, they preferred the run, even if they were forced to peck on a button before the gate to the run would open. The same method showed that hens have a strong desire to lay their eggs in a sheltered nest - impossible in the standard battery cage. As a result of this and other research, the European Union is now in the process of phasing in the most significant reforms of factory farming ever introduced. By 2012, hundreds of millions of hens will have more space, perches, and nest boxes, and veal calves and sows will no longer be housed in individual pens too narrow for them even to turn around, or walk a few steps.

In other areas of animal abuse, progress is slowly being made. Thanks to patient lobbying and dramatic campaigns against cosmetic giants like Revlon, developing methods of product testing that do not use animals has become respectable science. This month, EU research commissioner Philippe Busquin announced a method that uses human blood cells instead of live rabbits to detect the presence of fever-causing agents in drugs. That will save up to 200,000 rabbits a year from experimentation. But the resources going into developing and validating alternatives to animal use are still very small, compared to those going into research that continues to use animals.

One indication of the acceptability of a cause is when its opponents try to blunt its appeal by saying that, "of course", they agree with some of the claims made by those whose further claims they wish to reject. In the spirit of Gladstone's chancellor, the 19th-century Sir William Harcourt's remark that "We are all socialists now", today everyone, from scientists who experiment on animals, to foxhunters like Roger Scruton, is an animal welfarist. Scruton even says, in his little book Animal Rights and Wrongs, that "a true morality of animal welfare" ought to begin from the premise that the way we now treat animals on factory farms is wrong. In America, Matthew Scully, a conservative Christian, a past literary editor of National Review and now speech writer to President Bush, has amazed his fellow conservatives by publishing Dominion: The Power of Man, the Suffering of Animals and the Call to Mercy, an eloquent polemic against abuse of animals, culminating with a devastating description of factory farming. It has won praise from such icons of the American right as Pat Buchanan and Charles Colson. And, in November, the voters of Florida, not known as the most progressive of US states, gave the American animal movement its first ever victory in a referendum on factory farming when they voted to ban stalls that prevent sows from turning around.

Still, no society is even close to giving equal consideration to the interests of all animals. The spread of western methods of intensive farming to China and other nations in the developing world is threatening to incarcerate billions more animals in factory farms. After 30 years, the most that can be said is that - at least in the developed world - we are beginning to move in the right direction.

· Peter Singer is a professor of bioethics at Princeton University, in New Jersey. His Animal Liberation was first published in 1975. His most recent book is One World. psinger@princeton.edu